Doctoral Degrees (Horticultural Science)

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Avocado seed physiology aspects.
(2021) Abdalla, Mamoun Ahmed Arabi.; Bertling, Isa.
The avocado seeds/seedling is needed as rootstock for other economic trees and loss of tress stand in orchids after establishment is of great commercial loss in avocado orchids around the worldwide and South Africa. The aim of this study was to evaluate and compare avocado seeds development of various seeds ages by investigating seeds germination percentage over three generations, as there is little information on avocado seeds growth and development, despite the importance of the seeds in avocado propagation. Seed harvesting was carried out over various developmental stages, from early fruit development to two-year-old seeds (Generation 1, 24 to 29 months after full bloom MAFB). Seed from current season (Generation 2, 12 to 17 MAFB) and newest seeds (Generation 3, 0 to 5 MAFB) of two cultivars (‘Hass’ and ‘Fuerte’) was analysed. Seed of three generations were analysed: ‘Hass’ Generation 1seed (seed from the oldest, commercially over-mature, fruit full bloom in July/ August 2017); Generation 2 (full bloom in July/ August 2018) and Generation 3 (full bloom in July/August 2019). Similarly, ‘Fuerte’ fruit of three generations were compared: from the avocado fruit, (Generation 1, full bloom in June/July 2017), to Generation 2 (full bloom in June/July 2018) to Generation 3 (full bloom in June/July 2019). Seed were extracted from fruit to determine seed parameters, such as germination percentage, seed viability, seed moisture content and seed respiration rate. Further, seed physiological parameters, such as cotyledonal sugars and starch concentrations, seed coat phenolic compound concentrations and polyphenol oxidase (PPO) concentrations were determined. Anatomical features of the seed coat, such as seed coat thickness and seed coat ultrastructure were also observed. In both cultivars, the germination percentage was higher in Generation 2, 12 to 18 MAFB), than in Generation 1, 24 to 29 MAFB) seed from June to September. From October to November Generation 3 (0 to 5 MAFB) had a higher germination percentage than Generation 2. Seed viability was higher in Generation 2 of both cultivars and lower for the Generation 1; similar results were found for the germination percentage, with seed from Generation 2 having a higher germination rate than seed from the Generation 1. Seed viability differed significantly between seed age, and the interaction between generations and months was statistically significant (P ˂ 0.001). The seed collected from fruit of the Generation 2 of both cultivars had a slightly higher moisture content and a higher germination percentage than the Generation 1. Seed moisture content ranged between 54.5 and 62.1 % in ‘Hass’ (Generation 2 seed age 12 to 15 MAFB), harvested in June to September, while the Generation 1 seed age 23 MAFB) seed had a lower moisture percentage (39.2%) in June. ‘Hass’ seed of (Generation 3 seed age 4 MAFB) harvested from October to November had a higher seed moisture than seed from (Generation 2,15 MAFB). ‘Fuerte’ seed showed a similar pattern with the highest moisture percentage (60.5%) in July and the lowest in June (33.2%). (Generation 2’ seed age 13MAFB seed had higher moisture percentages than Generation 1 from June to September, and (Generation 3, 3 MAFB) had higher moisture percentage than Generation 2. Seeds respiration rate, determined following fruit harvest, decreased over the time. Generally, Generation 2 respired more than the Generation 1, from June to September. From October to November 2019 the Generation 3 respired more than Generation 2 seed. The respiration rate of seed extracted from June to September 2018 Generation 2 declined rapidly. The Generation 3 (collected October to November 2019, seed age 4 to 5 MAFB) were characterized by a higher respiration rate than seed of Generation 2, seed age 16 to 17 MAFB; therefore, younger seeds generations respired more than older ones. It is concluded that the contribution of seeds respiration rate to avocado whole fruit respiration decreases with development over the time. The ability of the avocado seed to germinate quickly and produce seedlings is dependent on the carbohydrate reserves in the cotyledons, which make up the bulk of the avocado seed. In seed coats of both cultivars, phenolic concentrations inhibited seed germination of Generation 1, probably due to the higher level of phenolic concentrations in older seed coats. Seed coats generally contained high amounts of phenolics (2.3 mg GAE*g-1 DM for ‘Hass’ and 2.02 mg GAE* g-1 DM for ‘Fuerte’). Seed extracted from Generation 1 fruit in June to September 2018, had a higher amount of seed coat phenolics than those from Generation 2 fruit. In fruit from October to November 2019 the Generation 3 seed coat had lower phenolic concentrations than Generation 2 seed coats, confirming that older seed coats contain more phenolics than younger seed coats. Germination percentages of Generation 3 seed were higher than those of Generation 2 seed. The high phenolic concentration in the seed coats seems to be aligned with the seed turning dark brown upon maturation, probably due to sufficient oxygen present in the fruit to allow phenolic oxidation of the seed coat; the seed coat becoming entirely brown and very thin, could, therefore, be used as an indication that the fruit has reached physiological maturity. Seed at this stage of maturation are, however, characterized by a low germination percentage, possibly due to the seed coat phenolic compounds interfering with germination. This is supported by the positive correlation between lower seed coat phenolic compound concentration and higher seed germination rate for both cultivars (r = 0.11, P ˂ 0.61). Seed coat thickness of Generation 1 (24 to 28 MAFB) and Generation 2 (12 to 16 MAFB) ‘Hass’ seed coats differed, with the younger seed generation displaying thicker seed coats than the older ones (0.51 versus 0.11 mm, respectively). In ‘Fuerte’, in June and July older seed coats Generation 1, 24 to 25 MAFB, respectively) were thicker than Generation 2 (12 to 13 MAFB) (0.46 and 0.15 mm, respectively. There was, however, negative relationship between seed coat thickness and germination percentage (r = -0.11). Polyphenol oxidase (PPO) and phenolic concentrations of avocado seed coats were also investigated in the seed coat of Generation 1and Generation 2 ‘Hass’ and ‘Fuerte’ seed. Polyphenol oxidase (PPO) and phenolic concentrations of the avocado seed coats of the two avocado cultivars of Generation 1, 27 to 26 MAFB) and Generation 2, 15 to 14 MAFB) respectively, seed coats were investigated. During the colder (winter) season (June-August), Generation 1, fully mature ‘Hass’ seed coats showed higher polyphenol oxidase (PPO) concentrations than seed coats from the Generation 2. From October to November the Generation 3 seed coat also had a lower PPO concentration than those of Generation 2. Generation 2 ‘Hass’ seed coats had relatively low PPO concentrations in June /July, when fruit were 12 to 14 MAFB, but PPO concentrations increased thereafter and remained at a higher-level until October/ November. Generation 1 ‘Fuerte’ seed coat had a similar PPO concentration during all investigated months. Phenolic compounds were present in seed coats of both avocado cultivars, with seed coats of older seeds containing a much higher phenolic concentrations than the seed coats of the newer generation. The seed (cotyledons plus embryo) sugar profile was dominated by the C7 sugar perseitol, followed by the C6 sugar, sucrose, while mannoheptulose and glucose were present in very small amounts. Perseitol was present in in both cultivars with 14 months-old ‘Hass’ (September) cotyledons containing 9.8 mg*g-1 DM and 15-months-old ‘Fuerte (September) containing 10.3 mg*g-1 DM. Avocado cotyledons were found to also be a large starch source, probably providing carbohydrates for seed development and germination. The Generation 2, 14 to 15 MAFB) of ‘Fuerte’ and ‘Hass’ had a higher starch concentration than the Generation 1, 26 to 27 MAFB) and similarly, Generation 3, 4 to 5 MAFB, respectively, had higher starch concentration than Generation 2 for both cultivars, indicating the use of this carbohydrate reserve to sustain embryo development. The highest concentration of starch in ‘Hass’ seeds was detected in August as 88.8% of seed DM (Generation 2, seed age 13 MAFB), while for ‘Fuerte’ seed the highest starch concentration was in August at 90.5% of seed DM (Generation 2, 14 MAFB). Starch seems, therefore, more related to avocado seed development than to avocado fruit growth and development. Delaying fruit harvest to October (seed age 16 to 18 MAFB) allows seed to fully mature and to continue accumulating sugars and starch. To improve percentage and velocity of germination, seeds were soaked in various concentrations of aqueous moringa leaf extract (MLE, 0, 2.5, 5.0 and 7.5 % w/v) over different periods (0, 10, 30 or 120 minutes). Younger seed were stronger affected by the increasing MLE concentration. Soaking in 2.5% MLE tended to enhance the germination percentage more so than the other MLE concentrations. The lowest germination percentage was determined for seeds soaked in 7.5% MLE for 120 minutes, indicating that younger seed (from10 to 12 months after fruit set ‘Fuerte’ fruit harvested April to June) should be used as ‘nurse seed’. Overall, this study revealed that avocado seed germination and development do not coincide with the commercial fruit harvesting period, the avocado fruit needs 15 to 18 months to change from its flowering blooming period to a full harvest, and seed age12 MAFB can germinate for both cultivars. The study further confirmed perseitol as the dominant free storage sugar that assists in seed development, while starch is also an important energy provider for the developing embryo.
The cascade of physiological events leading to chilling injury : the effect of post-harvest hot water and molybdenum applications to lemon (citrus limon) fruit.
(2012) Mathaba, Nhlanhla.; Bertling, Isa.; Bower, John Patrick.
New emerging markets such as Japan and the United States require cold sterilisation of South African citrus fruit as a phytosanitary standard against fruit fly. However, citrus fruit are chilling susceptible, with lemons being the second-most chilling susceptible after grapefruit. Chilling injury is a physiological rind disorder; the occurrence of which is despite its prevalence in horticultural commodities, not well understood. Therefore, the aim of this study was to investigate physiological compounds regulating chilling susceptibility or resistance in citrus fruit, with special emphasis on lemons. Furthermore, the potential of hot water dips or “molybdenum soaks” to maintain a certain level of physiological compounds which determine manifestation of chilling injury symptoms in citrus fruit was investigated. Moreover, it was attempted to create an understanding of the order in which physiological compounds mitigate chilling injury. Lemon fruit from different farms known to be chilling susceptible or resistant were obtained during the 2007 and 2008 harvest season. Thereafter, fruit were treated by soaking for 30 min in 1μM NaMo04.2H20 solution followed by a 2 min HWD 47 or 53°C. Treated fruit were waxed, weighed and stored at -0.5°C for up to 28 days and sampled for chilling injury evaluation 7, 14, 21, or 28 days into cold storage. A second evaluation was carried out five days after withdrawal from cold storage to allow development of chilling injury symptoms as a shelf-life simulation. After the second evaluation fruit were peeled, peel freeze-dried, milled using mortar and pestle and stored at -21°C for further physiological analysis. Freeze-dried peel was analysed for soluble sugars (glucose, fructose, sucrose), vitamin C (ascorbic acid), vitamin E (α-tocopherol), β-carotene, polyamines (putrescine, spermine, spermidine), specific flavanones (naringin and hesperidin) using HPLC-UV-Vis detector and proline, total antioxidant assays (FRAP, ABTS, DPPH), total phenolics, total flavonoids, lipid peroxidation using spectrophotometry, as well as for the heat shock protein (HSP70) using electrophoresis and silver-staining. Chilling susceptibility of lemon fruit varied with fruit source; those sourced from Ukulinga and Eston Estates were chilling resistant, while fruit from Sun Valley Estates showed chilling injury symptoms after 28 days of cold storage plus five days shelf-life. Furthermore, hot water dips (HW) 53°C, 1 μM Molybdenum (Mo) and 10 μM Mo plus HW 53°C significantly reduced chilling injury symptoms compared with the control and HW 47°C. In addition, Sun Valley Estates fruit also showed higher fruit weight loss compared with non-chilling resistant lemons. The alignment of higher fruit weight loss during storage with chilling susceptibility ascertains the use of weight loss as a non-destructive parameter for chilling susceptibility. With respect to flavedo sugars, glucose was found to be the dominant soluble sugar with multi-functional roles during cold storage. This plays a significant role in mitigating cellular stress. Chilling susceptible lemons from Sun Valley Estates had low flavedo glucose concentrations and, therefore, little conversion of glucose to ascorbic acid was possible resulting in a low antioxidant capacity. However, treatments with HW 53°C and Mo soaks seemed to enhance the enzymatic conversion of glucose to ascorbic acid leading to a higher antioxidant capacity in the flavedo of such treated fruit. Furthermore, glucose also feeds into the pentose phosphate pathway which is coupled with the shikimate pathway synthesizing secondary metabolites, especially of the phenolics group. The decrease in glucose was aligned to the levels of total phenolics, but not to that of β-carotene, naringin and hesperidin through 28 days into cold storage period. Moreover, as glucose also feeds into shikimate pathway, simultaneously an increase in proline flavedo concentration was observed. Proline is an antioxidant synthesized from glutamate; as cellular glucose decreases so does the total antioxidant capacity during cold storage. Ascorbic acid is a dominant and potent antioxidant in lemon flavedo as proven with the FRAP, ABTS and DPPH assays. Chilling resistant fruit have significantly higher ascorbic acid conversion. Furthermore, ascorbic acid also acts to generate the α-tocopheroxy radical to further important membrane-bound antioxidant, vitamin E (α-tocopherol equivalent). Furthermore, the DPPH assay was found to be effective in quantifying total antioxidants in lemon flavedo since it detects both lipophilic and hydrophilic antioxidants compared with the ABTS and FRAP assays which are bias to the estimation of liphophilic or hydrophilic antioxidants, respectively. The hot water and molybdenum treatments increased total antioxidants (DPPH assay) with reduced lipid peroxidation 7 days into cold storage and therefore, reduced chilling symptoms in fruit from Sun Valley Estates. The capacity of antioxidant to scavenge reactive oxygen species (ROS) was increased during cold storage and membrane stability significantly improved. Furthermore, putrescine as low valency polyamine was reduced as such compound acted as precursor to the synthesis of the high valency polyamines, spermine and spermidine. Chilling susceptible lemons from Sun Valley Estates showed increased soluble-conjugated polyamines as a response to stress. Furthermore, HW 53°C, 1 μM Mo and 10 μM Mo plus HW 53°C significantly increased the protein concentration and, therefore, likely also the occurrence of proteins with 70kDa (as estimator of HSP70). Additionally, the concentration of conjugated high valency polyamines was also increased, resulting in reduced chilling injury symptoms. The effect of ROS has only been viewed as damaging, while recently their role has also been viewed as stress acclamatory signalling compounds when produced concentrations below critical damaging threshold. Therefore, hot water dips seems to signals synthesis of total protein which include HSPs which then act throughout cold stress to protect other protein and channel other damaged proteins towards proteolysis. While molybdenum increased ROS production below damaging critical threshold, with ROS signalling stress acclimation by further signalling production of bioactive compound with antioxidant properties.
Induced polyploidy as a tool for the development of novel South African indigenous crops.
(2015) Hannweg, Karin Fiona.; Bertling, Isa.
Polyploidy is a naturally-occurring phenomenon in plants and has been reported as an important pathway for evolution and speciation; it is estimated that a large percentage of flowering plants are polyploid in origin. Although the first plant polyploid was discovered over a century ago, the genetic and evolutionary implications of polyploidy have not been fully elucidated. On a more practical level, there are many opportunities for utilizing induced polyploidy as a valuable tool in traditional plant breeding programmes. South Africa has the highest recorded plant species density in the world, however, many of these species have only marginal potential due to size and other constraints. Induced polyploids may be expected to exhibit one, or more, of the following characteristics resulting in the improvement or the development of new economically important plants: larger tuber, rhizome or root size; increased flower or fruit size; enhanced flower colour intensity, improved drought tolerance, increased bio-mass; improved photosynthetic capacity; larger and/or thicker leaves; dwarfism; increased secondary metabolite production, e.g. medicinal compounds. Several plant species (Crocosmia aurea, Tetradenia riparia, Siphonochilus aethiopicus and Plectranthus esculentus) were selected for the induction of polyploidy and various horticultural characteristics evaluated. Methods for the successful induction of polyploidy were developed for all selected species. By evaluating various horticultural characteristics of the induced polyploids it was determined that flower size, plant vigour and nematode resistance, as well as essential oil content and bioactivity could be significantly improved in all tested species. Induced polyploidy could, therefore, have a significant impact on the development of economically-viable novel crops indigenous to southern Africa.
Management of avocado postharvest physiology.
(2011) Blakey, Robert John.; Bower, John Patrick.; Bertling, Isa.
Avocados are an important horticultural crop in South Africa, especially in the provinces of KwaZulu-Natal, Mpumalanga and Limpopo. The distance to traditional export markets, phytosanitary restrictions to lucrative markets such as China, the USA and Japan and increased competition in the European market have challenged the South African avocado industry. The industry has responded with improved logistics and shipping, a co-ordinated market access program and a global system to co-ordinate exports of avocados to the European market. To remain competitive on the global market, further improvements and innovations are required to improve the efficiency of postharvest operations. These improvements and innovations should be guided by a greater understanding of postharvest physiology. Avocados are a relatively new export crop, so there is still much to be learnt about avocado postharvest physiology and the optimisation of postharvest management. In this regard, reduced temperature storage (1°C) and modified humidity packaging (MHP) were investigated for their effect on fruit physiology and quality, the effect of a water- and ABA-infusion on ripening was examined and the effect of a cold chain break on fruit physiology and quality determined; near-infrared spectroscopy was also examined for its potential for its use in the avocado industry. As an initial study, the relationships between individual sugars, protein and oil were studied to understand the changes in avocado fruit during ripening. It was found that mannoheptulose and perseitol were the predominant sugars at harvest, but declined to very low levels during the first 10 days postharvest. The concentrations of glucose and fructose increased, while sucrose declined slightly during ripening. The concentration of protein increased sigmoidally during ripening, reflecting the increase in the ripening enzymes, particularly cellulase and polygalacturonase. The oil content fluctuated slightly during ripening. It is suggested that mannoheptulose and perseitol are important carbon and energy sources during ripening. Glucose concentration was also found to increase earlier in fast ripening fruit compared to slow ripening fruit, which is related to increased cellulase activity and may be related to the ABA functioning. Thereafter, storage and ripening trials in two consecutive seasons showed that 1°C storage and the use of MHP for 28 days reduced mass loss, water loss, ethylene production, respiration, softening and heptose consumption, without appreciably affecting fungal rots, physiological disorders or external chilling injury, compared to fruit stored at 5.5°C and regular atmosphere respectively. Also, the storage of fruit in MHP delayed the rise in the activity of cellulase during ripening, compared to fruit not stored in MHP, but there was no significant difference in the peak activity of cellulase, polygalacturonase or pectin methylesterase. In a separate experiment, fruit ripening was significantly affected by the infusion of ABA in an aqueous solution. Water slightly reduced the variation in ripening while ABA reduced the time to ripening and the variation; it is suggested that water stress and ABA are intrinsically involved in the ripening processes and may act as a ripening trigger. The water concentration in fruit was measured non-destructively using reflectance NIR; this model was used to determine the maturity of fruit and the loss of water during cold storage. In the cold chain break experiment, it was found that although fruit recovered after a cold chain break, in terms of ethylene production and respiration, there was a loss in quality because of severe shrivelling as a result of increased water loss. Fruit that were stored at 1°C were generally of a better quality at ripeness, if the cold chain was broken, compared to fruit stored at 5.5°C. In a follow-up experiment, it was found that significant changes occurred in avocado physiology over a 6h period. The respiration rate of fruit significantly increased after 4h at room temperature and mannoheptulose declined by 32% in control fruit and by 16% in ethephon-treated fruit after 6h. This demonstrates the potential for quality loss in a short amount of time. Furthermore, a model of avocado ripening is proposed, outlining the role of water, ABA, ethylene, respiration, ripening enzymes and individual sugars. This study has contributed to the understanding of avocado postharvest physiology and should aid in better management of avocados for improved fruit quality and consumer satisfaction.
Methyl jasmonate and salicylic acid enhance chilling tolerance in lemon (citrus limon) fruit.
(2013) Siboza, Xolani Irvin.; Bertling, Isa.; Odindo, Alfred Oduor.
South African ‘Eureka’ lemon fruit must be exposed to chilling temperatures (± 0.6°C) as a mandatory quarantine treatment against insect pests for all its overseas markets. Chilling lemon fruit at such temperatures may develop chilling injury (CI) symptoms on the flavedo. This negative effect on fruit quality reduces fruit marketability. This study evaluated postharvest factors influencing physiological, biochemical and ultra-structural mechanisms involved in alleviating CI in lemon fruit. It was hypothesised that treatment with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by maintaining cellular integrity and inducing synthesis of enzymatic and non-enzymatic antioxidants. Furthermore, fruit susceptibility to CI was associated with the source of fruit. Lemon fruit were harvested from three locations representative of moderate subtropical, warm temperate and cool subtropical environments. Harvested fruit were treated either with 10 μM MJ, 2 mM SA or 10 μM MJ plus 2 mM SA, stored either at -0.5, 2 or 4.5°C for 0, 7, 14, 21, or 28 days and afterwards transferred to 23°C for a week as shelf-life simulation. Thereafter, fruit were evaluated for alterations in physiological, biochemical and ultra-structural features involved in the manifestation of CI symptoms. Chilling damage was more severe in untreated lemon fruit than in treated lemon fruit. Storing lemon fruit at 4.5°C accelerated the manifestation of CI symptoms more so than at 2°C while storage at -0.5°C delayed the manifestation of CI symptoms. Lemon fruit of moderate subtropical origin were more chilling-tolerant than lemon fruit of warm temperate and cool subtropical origin. Treatment with 10 μM MJ plus 2 mM SA significantly (P < 0.05) improved chilling tolerance in lemon fruit. This treatment effectively maintained membrane integrity, thereby retarding electrolyte leakage and membrane lipid peroxidation as well as mass loss and respiration rate. Treatment with 10 μM MJ plus 2 mM SA was also effective in enhancing the antioxidant concentrations of vitamin E and carotenoids. The production of these antioxidants could have been part of a defence system against chilling damage, reducing CI and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA enhanced the concentration of compounds involved in chilling resistance, such as proline, soluble sugars, ascorbic acid and total phenolics as well as the enzyme phenylalanine ammonia-lyase (PAL). The enhancement of the defence mechanisms may have played a role in enhancing chilling tolerance in lemon fruit. The treatment also inhibited certain enzymes involved in tissue browning, such as peroxidase (POD) which might have contributed to delaying manifestation of symptoms. Polyphenol oxidase (PPO) was found to not be a good biochemical marker of the occurrence of CI. Treatment with 10 μM MJ plus 2 mM SA appeared to be able to enhance chilling tolerance in lemon fruit by maintaining the ultra-structure of the cuticle, cell wall integrity, cell membrane of parenchyma cells of the flavedo. This treatment also preserved the mineral nutrients of the flavedo (carbon, oxygen, phosphorus, potassium, calcium, magnesium, sulphur, sodium, silicon and aluminium) during cold storage. This could have played a role in protecting the fruit against chilling stress and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA reduced ROS production, while the activity of enzymatic antioxidants such as catalyse (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), and accumulation of essential proteins was enhanced. This increase in activity of enzymatic antioxidants and the presence of stress-responsive proteins in the lemon flavedo could have been directly involved in enhancing chilling tolerance. The CI symptoms were accompanied by an increase in membrane permeability, membrane lipid peroxidation as well as phospholipase D (PLD) and lipoxygenase (LOX) activity; however, treatment with 10 μM MJ plus 2 mM SA effectively reduced the membrane permeability, membrane lipid peroxidation, and PLD and LOX activity induced by the cold treatment. This could have contributed to the efficacy of 10 μM MJ plus 2 mM SA in inhibiting the manifestation of CI symptoms. Treatment with 10 μM MJ plus 2 mM SA enhanced flavedo total antioxidant capacity measured by ferric reducing ability of plasma; 2,2-diphenyl-1-picrylhydrazyl; 2,2-azinobis (3-ethyl-benzothiazoline-6-sulfonic acid) and the oxygen radical absorption capacity assays. The enhancement of antioxidant capacity in lemon flavedo could have contributed to the fruit’s chilling tolerance. Therefore, the effect of 10 μM MJ plus 2 mM SA treatment, enhancing chilling tolerance, may be attributed to its ability to enhance enzymatic and non-enzymatic antioxidants; activate essential proteins and mitigate the effect of ROS accumulation. With the use of 10 μM MJ plus 2 mM SA treatments, the South African citrus industry will be able to meet the quarantine temperature requirements for exportation of lemon fruit whilst reducing economic losses, depending on the preharvest conditions experienced by the fruit in each shipment.
Optimization of flower and seed crop production in temperate eucalyptus orchards in South Africa through site selection and environmental manipulation.
(2014) Gardner, Robin Arthur William.; Bertling, Isa.; Savage, Michael John.
Temperate eucalypts are an important part of the commercial forestry landscape in South Africa, comprising approximately 50% of the total Eucalyptus planted area. The majority of the commercial temperate eucalypts grown in South Africa are reticent, shy flowerers, and subsequently erratic seed producers. Disadvantages associated with sub-optimum (inconsistent and sparse) flowering in Eucalyptus orchards include decreased levels of out-crossing and compromised quantity and (genetic) quality of the seed produced. Genotype, physiological age and a range of environmental factors are known to influence flower bud production in temperate eucalypts. To date, winter cold and paclobutrazol (PBZ), a plant growth regulator, remain the most effective treatments for encouraging early and prolific flowering in temperate eucalypts. Disadvantages associated with the use of PBZ in the outdoor environment include the toxicity and recalcitrant nature and persistence of the chemical in soils, the high cost of PBZ and its orchard application, and the need to re-apply the chemical approximately every five years. The main aim of this study was to provide a practical solution to the problem of shy flowering and seed crop production in important temperate Eucalyptus species in South Africa. The study focused on investigating key environmental factors associated with optimum flower bud production in temperate eucalypts. The resultant data were used to achieve optimization of flower bud production in temperate eucalypts in the summer rainfall forestry areas of South Africa, via informed site selection and/or manipulation of the environmental conditions. A subsidiary aim of the project was to lessen the dependency on PBZ for achieving satisfactory flowering levels in temperate eucalypts via improved site-orchard matching and environmental manipulation. The key objectives in the study included definition of summer rainfall area site conditions for maximal floral bud production in two important species, viz. E. nitens and E. smithii, investigation of the effects of optimum flowering environmental conditions on post-initiation floral development and seed maturation in temperate eucalypts, and development of a method for supplementing winter cold and increasing flower bud production in orchards located at marginal winter chilling sites.
Special carbohydrates of avocado : their function as 'sources of energy' and 'anti-oxidants'.
(2009) Tesfay, Samson Zeray.; Bertling, Isa.
There is increasing interest in special heptose carbohydrates, their multifunctional roles from a plant physiological view point in fruit growth and development as well as in the whole plant in general due to their potential in mitigating photo-oxidative injury to the whole plant system and the image of avocado as ‘health fruit’. Studies have been carried out to investigate the role of avocado heptoses, rare carbohydrates predominantly produced in avocado. Several authors have documented various research findings and speculated on multifunctional roles of avocado special sugars. However, few reports have made an attempt to elucidate the multifunctional roles of avocado heptose carbohydrates as: ‘sources of energy’, storage and phloem-mobile transport sugars, and precursors for formation of antioxidants. Assessing the avocado carbohydrates over the plant growth and development during ontogeny may, therefore, offer clues to better understand whole plant behaviour. Plant sampling was carried out over different developmental stages. Using plants grown in the light versus etiolated seedlings; sugar determinations were also done to determine what sugar is produced from which storage organs. The sugars were extracted and analysed by isocratic HPLC/RID. The embryo had 47.11 % hexose and 52.96 % heptose sugars. The seed, however, also released significant amounts of D-mannoheptulose (7.09 ± 1.44 mg g-1 d. wt) and perseitol (5.36 ± 0.61 mg g-1 d. wt). Similarly fruit and leaf tissues had significant amounts of heptoses relative to hexoses at specific phenological stages. In postharvest ‘readyto-eat’ fruit the following carbohydrate concentrations were as follows:exocarp heptoses 13 ± 0.8; hexoses 4.37 ± 1.6 mg g-1 d. wt, mesocarp heptoses 8 ± 0.2; hexoses 3.55 ± 0.12 mg g-1 d. wt), seed heptoses (only perseitol) 13 ± 1.1; hexoses 5.79 ± 0.53 mg g-1 d. wt. The results of this experiment was the first to demonstrate that the heptoses D-mannoheptulose, and its polyol form, perseitol, are found in all tissues/organs at various phenological stages of avocado growth and development. Secondly, heptoses, as well as starch are carbohydrate reserves that are found in avocado. The heptoses, beyond being abundantly produced in the avocado plant, are also found in phloem and xylem saps as mobile sugars. The study also presents data on the interconversion of the C7 sugars Dmannoheptulose and perseitol. It is deduced that D-mannoheptulose can be reduced to perseitol, and perseitol can also be oxidized to D-mannoheptulose by enzymes present in a protein extract of the mesocarp. The potential catalyzing enzyme is proposed to be an aldolase, as electrophoretic determinations prove the presence of such an enzyme during various stages of development in various plant organs. Avocado heptoses play an important role in plant growth and development and in fruit in particular. Moreover, they are reported as sources of anti-oxidants, and contribute significantly to fruit physiology if they function in coordination with other anti-oxidants in fruit tissues. To evaluate the presence of anti-oxidant systems throughout avocado fruit development, various tissues were analysed for their total and specific anti-oxidant compositions. Total anti-oxidant levels were found to be higher in the exocarp and in seed tissue than in the mesocarp. While seed tissues contained predominantly ascorbic acid (AsA) and total phenolics (TP), the anti-oxidant composition of the mesocarp was characterised by the C7 sugar, D-mannoheptulose. Among the anti-oxidant enzymes assayed, peroxidase (POX) and catalase (CAT) were present in higher concentrations than superoxide dismutase (SOD) in mesocarp tissue. Different anti-oxidant systems seem to be dominant within the various fruit tissues. Carbohydrates are the universal source of carbon for cell metabolism and provide the precursors for the biosynthesis of secondary metabolites, for example via the shikimic acid pathway for phenols. The preharvest free and membrane-bound phenols, catechin and epicatechin, are distributed differently in the various fruit tissues. Membrane-bound and free phenols also play a role as anti-oxidants, with free ones being more important. KSil (potassium silicate) application to fruit as postharvest treatment was used to facilitate the release of conjugates to free phenols via lysis. This treatment improved fruit shelf life. Western blotting also revealed that postharvest Si treatment affects the expression of enzymatic anti-oxidant-catalase (CAT). Overall the thesis results revealed that C7 sugars have anti-oxidant properties and that D-mannoheptulose is the important anti-oxidant in the edible portion of the avocado fruit. Dmannoheptulose is furthermore of paramount importance as a transport sugar. Perseitol on the other hand acts as the storage product of D-mannoheptulose, which can be easily converted into D-mannoheptulose.
Yield and quality parameters of tomato cultivars as affected by different soilless production systems and beneficial micro-organisms.
(2013) Maboko, Martin Makgose.; Bertling, Isa.; Du Plooy, Christian Phillipus.
Most tomato cultivars used for commercial food production are imported into South Africa. Optimal growing conditions for these specific cultivars need to be determined, as wrong cultivar choices can lead to great financial losses. Lack of information on selecting well-performing cultivars may lead to lower yield or unacceptable fruit quality. Information on the performance of tomato cultivars under South African conditions, utilizing plastic tunnels or shadenet structures under soilless cultivation is still very limited. Soilless cultivation of vegetables is becoming a preferable over in-soil cultivation due to the improved yield and quality of produce, efficient water and nutrients usage by the crop; furthermore, the grower can regulate nutrient solution, electrical conductivity and pH of the nutrient solution. To identify the optimal system for growing tomatoes hydroponically, the performance of four tomato cultivars (‘FA593’, ‘Miramar’, ‘FiveOFive’ and ‘Malory’) under different growing conditions was evaluated: directly planted in soil under 40% shadenet with drip irrigation, a closed hydroponic system under 40% shadenet, an open bag system under 40% shadenet, or an open-bag system in a temperature controlled as well as a non-temperature controlled tunnel. The study revealed that ‘Miramar’ performed better than the other cultivars in all production systems, with the exception of soil cultivation where there were no differences amongst the four cultivars. Fruit cracking was found to be directly correlated with fruit size, as the large-sized cultivars ‘Malory’ and ‘FA593’ were more susceptible than the other two cultivars. Plants grown under shadenet were prone to fruit cracking and raincheck as well as early blight. Higher yields were obtained when plants were produced in the open bag system under temperature controlled conditions and in the closed system under shadenet. Growing tomatoes in the non-temperature controlled tunnel resulted in high incidences of fruit cracking, poor yield and pre-mature fruit ripening probably due to high and fluctuating temperatures under such conditions. The average marketable yield was 88% and 59% of the total yield in the temperature controlled and non-temperature controlled tunnels, respectively. A further experiment was carried out to improve yield and quality of tunnel tomatoes using beneficial micro-organisms, i.e., arbuscular mycorrhiza fungi (AMF) at different nutrient concentrations. Tomato seedlings were treated with Mycoroot™ containing four mycorrhiza species (Glomus etunicatum, Paraglomus occultum, Glomus clarum and Glomus mossea) at transplanting and subsequently transferred to either a temperature controlled or a non-temperature controlled tunnel under the recommended (100%) or reduced (75 and 50%) nutrient concentrations. Sawdust was used as a growing medium in this experiment. Application of AMF neither enhanced plant growth, yield, nor fruit mineral nutrient concentrations; although fruit Mn and Zn concentrations in the temperature controlled tunnel increased significantly following AMF application. Plants grown in the non-temperature controlled tunnel had significantly poorer plant growth, and lower yield and lower fruit mineral concentrations, compared with fruit from plants in the temperature controlled tunnel. Tomato plants in the non-temperature controlled tunnel had higher levels of micro-elements in leaf tissue, compared with those in the temperature controlled tunnel. The highest yields were obtained from plants fertigated with 75% of the recommended nutrient concentration, as compared with the 100 and 50% nutrient concentrations. When coir was subsequently used as the growing medium, Mycoroot™ applied at seeding and transplanting did not enhance mycorrhizal colonization or fruit quality. Growing tomatoes under reduced nutrient supply reduced the total soluble solids in the juice of the fruit, but improved total and marketable yield, as well as the number of marketable fruit. This effect was more substantial in the temperature controlled than in the non-temperature controlled tunnel. Fruit firmness and leaf chlorophyll concentrations were significantly higher in plants grown in the temperature controlled tunnel. Growing tomatoes in sawdust improved the leaf Mn and Ca concentration over that of tomato plants grown in coir. Mycorrhizal colonisation did not have a beneficial effect on tomato yield and quality. The study indicated that cultivar selection was important in obtaining the highest yield and quality of tomato using the closed hydroponic system under shadenet and the open bag hydroponic system in the temperature controlled tunnel. Temperature controlled tunnels with a pad–and-fan cooling system are still an effective way of cooling the tunnel environment which resulted in high yield and high quality of tomatoes with a higher fruit mineral content than that obtained under non-temperature controlled conditions where only natural ventilation is relied on. Results also demonstrated that mycorrhizal colonization in soilless condition has limited beneficial effects in allowing for better nutrient uptake and thereby for improved yield and quality of tomatoes. Further studies, including different media, nutrient composition and concentrations, need to be carried out to investigate the possible causes of AMF failure to improve yield, despite good AMF root colonization.

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Browsing Doctoral Degrees (Horticultural Science) by Author "Bertling, Isa."